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The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex (a metal carbonyl, e.g. nickel carbonyl); in this situation, carbon is triple-bonded to oxygen : C≡O.
The remainder of this article concerns itself with the organic chemistry definition of carbonyl.
A carbonyl group characterizes the following types of compounds (where -CO denotes a C=O carbonyl group):
Oxygen is more electronegative than carbon, and thus pulls electron density away from carbon to increase the bond's polarity. Therefore, the carbonyl carbon becomes electrophilic, and thus more reactive with nucleophiles. Also, the electronegative oxygen can react with an electrophile; for example a proton in an acidic solution or other Lewis Acid.
The alpha hydrogen in a carbonyl compound is much more acidic (~1030 times more acidic) than a typical CH bond. For example the pKa values of acetaldehyde and acetone are 16.7 and 19, respectively.
Amides are the most stable of the carbonyl couplings due to their high resonance stabilization between the Nitrogen-Carbon and Carbon-Oxygen bonds.
Other important reactions include:
α,β-Unsaturated carbonyl compounds
α,β-Unsaturated carbonyl compounds are an important class of carbonyl compounds with the general structure Cβ=Cα−C=O. In these compounds the carbonyl group is conjugated with an alkene (hence the adjective unsaturated), from which they derive special properties. Examples of unsaturated carbonyls are acrolein, mesityl oxide, acrylic acid and maleic acid. Unsaturated carbonyls can be prepared in the laboratory in an aldol reaction and in the Perkin reaction. The carbonyl group, be it an aldehyde or acid, draws electrons away from the alkene and the alkene group in unsaturated carbonyls are therefore deactived towards an electrophile such as bromine or hydrochloric acid. As a general rule with unsymmetric electrophiles hydrogen attaches itself at the α position in an electrophilic addition. On the other hand, these compounds are activated towards nucleophiles in nucleophilic addition.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Carbonyl". A list of authors is available in Wikipedia.|